1. Field of the Invention
This invention relates to a rubber composition for use in a tread of high running-performance pneumatic tire, and more particularly to a rubber composition for use in a tread of a high running-performance pneumatic tire having good wear resistance and fracture properties and excellent wet skid property, ice skid property and dry gripping property by including a low molecular weight butadiene polymer as a rubber ingredient.
2. Description of the Related Art
Recently, the demand on a high-speed running stability of a vehicle becomes severer and hence a rubber material for use in a tread of a tire is strongly required to have excellent wet skid resistance, ice skid resistance and dry gripping property in addition to fracture properties such as wear resistance and the like required from the old time.
However, it is difficult to simultaneously satisfy these properties, and particularly the wet skid resistance, ice skid resistance, dry gripping property and fracture properties are conflicting with each other.
In order to solve such a problem, there have hitherto been proposed various block copolymers as a rubber composition satisfying the above properties at a wide temperature range.
For example, rubber compositions using a block copolymer between high Tg polymer and low Tg polymer such as a block copolymer between styrene-butadiene copolymer having a high bound styrene content and styrene-butadiene copolymer having a low bound styrene content, a block (co)polymer between styrene-butadiene copolymer having a high vinyl content and (styrene) butadiene (co)polymer having a low vinyl content, or the like are disclosed in Japanese Patent laid open No.57-92014, No. 57-200413, No.61-55135, No.61-141742, No.61-231016, No. 62-240347, No.63-112648, No. 63-137945, No. 63-90522 and No.1-297412 and Japanese patent Application Publication No.63-60768, No.63-60769, No.63-39015 and No. 3-80165 and the like.
In these rubber compositions, it is attempted to improve the wet skid resistance, ice skid resistance and low fuel consumption by widening the Tg range of the polymer, but the dry gripping property largely exerting on the running stability at high speeds is not yet sufficient.
Further, a rubber composition obtained by adding a process oil to the block polymer between high Tg polymer and low Tg polymer, and a rubber composition obtained by coupling the high Tg polymer with the low Tg polymer and adding a process oil thereto are disclosed In Japanese Patent Application Publication No. 49-37415, Japanese Patent laid open No. 62-143959 and the like.
These rubber compositions aim at the improvement of ride comfortability as a tire for a vehicle or vibration damping properties as industrial products, but the wear resistance, fracture properties, wet skid resistance and ice skid resistance as the tire for the vehicle are still insufficiently satisfied.
Moreover, rubber compositions aiming at the improvement of processability, low fuel consumption, fracture properties and gripping force by blending high polymer with low polymer are disclosed in Japanese Patent Application Publication No. 59-52664 and Japanese Patent laid open No. 58-147442, No. 58-147443, No. 60-240746, No. 61-203145, No. 62-135506, No. 64-16845 and the like. However, the wet skid resistance, ice skid resistance, dry gripping property and wear resistance are still insufficiently satisfied.
In Japanese Patent laid open No. 1-197541 is disclosed a rubber composition aiming at the improvement of dry gripping property and wear resistance by widening Tg range of a high polymer and adding a low polymer thereto. However, the polymer for this composition is a styrene-butadiene copolymer obtained by emulsion copolymerization, so that the improvement of ice skid resistance is insufficient.